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United States Patent |
6,120,925
|
Kawatsu
,   et al.
|
September 19, 2000
|
Apparatus for and method of reducing concentration of carbon monoxide
and fuel-cells generator system with such apparatus
Abstract
The structure of the present invention enables all catalysts packed in a
cooling layer to be kept in an active temperature range, thereby
sufficiently reducing the concentration of carbon monoxide included in a
hydrogen-rich gas. A supply of water is fed through a water inlet pipe 40
to a selective CO oxidizing unit 34 of a reformer 30. The heat of
vaporization of the supplied water directly cools down selective CO
oxidizing catalysts 50 stored in the selective CO oxidizing unit 34. This
enhances the cooling efficiency and enables all the selective CO oxidizing
catalysts 50 stored in the selective CO oxidizing unit 34 to be maintained
in the active temperature range, thus sufficiently reducing the
concentration of carbon monoxide included in a resulting gaseous fuel.
Inventors:
|
Kawatsu; Shigeyuki (Susono, JP);
Taki; Masayoshi (Kounan, JP)
|
Assignee:
|
Toyota Jidosha Kabushiki Kaisha (Toyota, JP)
|
Appl. No.:
|
935062 |
Filed:
|
September 22, 1997 |
Foreign Application Priority Data
Current U.S. Class: |
429/40; 48/127.7; 422/196; 429/17; 429/19; 429/26; 429/34 |
Intern'l Class: |
H01M 008/04 |
Field of Search: |
422/171-77,196
429/17,19,34,26
48/127.7
|
References Cited
U.S. Patent Documents
4554223 | Nov., 1985 | Yokoyama | 429/20.
|
5271916 | Dec., 1993 | Vanderborgh | 423/246.
|
5432021 | Jul., 1995 | Wilkinson et al. | 429/17.
|
5658681 | Aug., 1997 | Sato | 429/13.
|
5843195 | Dec., 1998 | Aoyama | 48/127.
|
5874051 | Feb., 1999 | Heil et al. | 422/171.
|
Foreign Patent Documents |
5-201702 | Aug., 1993 | JP.
| |
7-185303 | Jul., 1995 | JP.
| |
7-196302 | Aug., 1995 | JP.
| |
Primary Examiner: Nuzzolillo; Maria
Assistant Examiner: Ruthkosky; Mark
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Claims
What is claimed is:
1. An apparatus for reducing concentration of carbon monoxide included in a
hydrogen-rich gas, which contains both hydrogen and carbon monoxide, said
apparatus comprising:
oxidizing gas introducing means for introducing an oxygen-containing
oxidizing gas to said hydrogen-rich gas;
an oxidizing unit having a catalyst that enables oxygen included in said
oxidizing gas to be bonded to said carbon monoxide included in said
hydrogen-rich gas preferentially over hydrogen included in said
hydrogen-rich gas; and
water supply means for applying a supply of water onto the catalyst itself.
2. An apparatus in accordance with claim 1, wherein said water supply means
comprises a plurality of water supply conduits that enable water to be fed
to said oxidizing unit via different pathways.
3. An apparatus in accordance with claim 2, wherein said catalyst in said
oxidizing unit is divided into a plurality of groups of catalyst having a
predetermined shape,
said plurality of groups of catalyst being arranged along a flow direction
of said hydrogen-rich gas,
said plurality of water supply conduits being arranged to enable water to
be fed to said plurality of groups of catalyst.
4. An apparatus in accordance with claim 1, said apparatus further
comprising:
temperature detection means for measuring a temperature of said oxidizing
unit; and
control means for regulating an amount of water supply by said water supply
means, thereby enabling said temperature of said oxidizing unit to be kept
within a predetermined range.
5. An apparatus in accordance with claim 1, said apparatus further
comprising:
carbon monoxide concentration detection means for measuring a concentration
of carbon monoxide included in said hydrogen-rich gas;
oxidizing gas supply control means for regulating an amount of oxidizing
gas supply by said oxidizing gas introducing means according to said
concentration of carbon monoxide; and
water supply control means for regulating an amount of water supply by said
water supply means according to said amount of oxidizing gas supply.
6. An apparatus in accordance with claim 5, wherein said water supply
control means comprises:
water supply calculation means for specifying said amount of water supply
by said water supply means in order to hold a predetermined ratio to said
amount of oxidizing gas supply.
7. An apparatus in accordance with claim 1, said apparatus further
comprising:
oxidative reaction detection means for detecting a progress of an oxidative
reaction in said oxidizing unit; and
control means for regulating an amount of water supply by said water supply
means according to said progress of said oxidative reaction.
8. An apparatus in accordance with claim 7, wherein said oxidative reaction
detection means comprises means for measuring a temperature of said
oxidizing unit, wherein the oxidative reaction detection means detects
said progress of said oxidative reaction based on said temperature of said
oxidizing unit.
9. An apparatus in accordance with claim 7, wherein said oxidative reaction
detection means comprises means for measuring a flow rate of said
hydrogen-rich gas, wherein said oxidative reaction detection means detects
said progress of said oxidative reaction based on said flow rate of said
hydrogen-rich gas.
10. An apparatus in accordance with claim 7, wherein said oxidative
reaction detection means comprises means for measuring a concentration of
carbon monoxide included in said hydrogen-rich gas, wherein said oxidative
reaction detection means detects said progress of said oxidative reaction
based on said concentration of carbon monoxide.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for and a method of reducing
the concentration of carbon monoxide and a fuel-cells generator system
with such an apparatus. More specifically the present invention pertains
to an apparatus for reducing the concentration of carbon monoxide included
in a hydrogen-rich gas, which contains hydrogen and carbon monoxide and
has a concentration of carbon monoxide lower than a concentration of
hydrogen, and also to a method of the same. The present invention further
pertains to a fuel-cells generator system with such an apparatus for
reducing the concentration of carbon monoxide.
2. Description of the Related Art
In a fuel-cells generator system, supplies of hydrogen and oxygen are
respectively fed to a fuel electrode and an oxidation electrode arranged
across an electrolyte, and an electromotive force is generated through an
electrochemical reaction on the electrodes. It is desirable to supply a
hydrogen-rich gas to the fuel electrode, in order to enhance the power
generation efficiency and prevents the air pollution.
A reformer is thus arranged before the fuel cells. The reformer reforms a
crude gaseous fuel, which includes a hydrocarbon, such as methanol, or an
alcohol as the primary component, to a hydrogen-rich gas in the presence
of a reforming catalyst. Even in the reformer of enhanced ability,
however, contamination of the reformed gas with a small amount of carbon
monoxide is inevitable. Existence of carbon monoxide in the reformed gas
fed to the fuel electrode poisons platinum (Pt) carried as a catalyst on
the fuel electrode, thereby lowering or unstabilizing the performance of
power generation.
A technique proposed for preventing such a phenomenon selectively oxidizes
carbon monoxide included in the reformed gas in the presence of a
catalyst, thus reducing the concentration of carbon monoxide included in
the gaseous fuel fed to the fuel electrode.
As disclosed in JAPANESE PATENT LAYING-OPEN GAZETTE No. 7-185303, another
proposed technique uses a cooling layer arranged in a catalyst layer with
the catalyst packed therein, thereby preventing the temperature of the
catalyst from exceeding an active temperature range. This technique
enables the catalyst to be kept in the active temperature range and
accordingly reduces the concentration of carbon monoxide to a level that
does not poison the catalyst on the electrodes in the fuel cells.
In the proposed technique, a coolant flowing through the cooling layer is a
liquid mixture of water and methanol, which is the reforming material.
This may cause part of the coolant to be vaporized in the cooling layer.
Vaporization of the coolant raises the pressure in the cooling layer and
prevents the coolant from flowing at a constant flow rate. The pulsative
flow of the coolant causes non-uniformity of temperature in the cooling
layer and thus prevents the whole catalyst layer from being cooled
homogeneously. As a result, all the catalysts packed in the catalyst layer
can not be kept within the active temperature range.
SUMMARY OF THE INVENTION
One object of the present invention is thus to enable all catalysts packed
in a cooling layer to be kept within an active temperature range, thereby
sufficiently reducing the concentration of carbon monoxide included in a
hydrogen-rich gas.
Another object of the present invention is to reduce the concentration of
carbon monoxide included in a gaseous fuel fed to a fuel electrode,
thereby enhancing the efficiency of power generation of a fuel-cells
generator system.
At least part of the above and the other related objects is realized by an
apparatus for reducing concentration of carbon monoxide included in a
hydrogen-rich gas, which contains both hydrogen and carbon monoxide. The
apparatus includes: oxidizing gas introducing means for introducing an
oxygen-containing oxidizing gas to the hydrogen-rich gas; an oxidizing
unit having a catalyst that enables oxygen included in the introduced
oxidizing gas to be bonded to the carbon monoxide included in the
hydrogen-rich gas preferentially over hydrogen included in the
hydrogen-rich gas; and water supply means for feeding a supply of water
into the oxidizing unit.
In the carbon monoxide concentration reduction apparatus of the present
invention (hereinafter referred to as the apparatus of the fundamental
structure), the oxidizing gas introducing means introduces the oxidizing
gas to the hydrogen-rich gas containing carbon monoxide. The catalyst in
the oxidizing unit enables oxygen included in the introduced oxidizing gas
to be bonded to the carbon monoxide included in the hydrogen-rich gas
preferentially over hydrogen. The water supply means feeds a supply of
water into the oxidizing unit. The oxidative reaction in the oxidizing
unit is exothermic and thereby vaporizes water fed from the water supply
means in the oxidizing unit. This structure allows the catalyst in the
oxidizing unit to be directly cooled down by the heat of vaporization.
The apparatus of the fundamental structure directly cools down the catalyst
in the oxidizing unit. This structure enhances the cooling efficiency and
enables all the catalyst in the oxidizing unit to be kept within an active
temperature range, thereby sufficiently reducing the concentration of
carbon monoxide included in the hydrogen-rich gas.
In the apparatus of the fundamental structure, the water supply means may
have a plurality of water supply conduits that enable water to be fed to
the oxidizing unit via different pathways. This structure increases the
area of the catalyst that receives a supply of water, thus homogeneously
cooling down the whole oxidizing unit and preventing non-uniformity of
temperature. This enables all the catalyst in the oxidizing unit to be
more readily kept within the active temperature range.
In the structure with the plurality of water supply conduits, the catalyst
in the oxidizing unit may be divided into a plurality of groups of
catalyst having a predetermined shape. In this structure, the plurality of
groups of catalyst are arranged along a flow direction of the
hydrogen-rich gas, and the plurality of water supply conduits are arranged
to enable water to be fed to the plurality of groups of catalyst. This
structure increases the area of the catalyst that receives a supply of
water by the number of water supply conduits, thus more effectively
preventing non-uniformity of temperature.
In accordance with one preferable application, the apparatus of the
fundamental structure further includes: temperature detection means for
measuring a temperature of the oxidizing unit; and control means for
regulating an amount of water supply by the water supply means, thereby
enabling the temperature of the oxidizing unit to be kept within a
predetermined range.
This preferable structure regulates the amount of water supply by the water
supply means and thereby controls the temperature of the oxidizing unit
measured by the temperature detection means within the predetermined
range. This structure enables the oxidizing unit to be kept within a
desired temperature range, that is, the active temperature range of the
catalyst in the oxidizing unit, thereby effectively reducing the
concentration of carbon monoxide included in the hydrogen-rich gas.
In accordance with another preferable application, the apparatus of the
fundamental structure further includes: carbon monoxide concentration
detection means for measuring a concentration of carbon monoxide included
in the hydrogen-rich gas; oxidizing gas supply control means for
regulating an amount of oxidizing gas supply by the oxidizing gas
introducing means according to the concentration of carbon monoxide; and
water supply control means for regulating an amount of water supply by the
water supply means according to the amount of oxidizing gas supply.
In this preferable structure, in response to an increase in concentration
of carbon monoxide in the hydrogen-rich gas, the oxidizing gas supply
control means increases the amount of oxidizing gas supply and enhances
the progress of the oxidative reaction, thereby reducing the concentration
of carbon monoxide included in the hydrogen-rich gas. The enhanced
progress of the oxidative reaction increases the heat generated by the
oxidative reaction. The water supply control means regulates the amount of
water supply according to the amount of oxidizing gas supply. This enables
the degree of cooling the oxidizing unit to be specified according to the
amount of heat generated by the oxidative reaction. This structure also
enables the oxidizing unit to be kept within a desired temperature range,
thereby effectively reducing the concentration of carbon monoxide included
in the hydrogen-rich gas.
In the apparatus of the above structure, it is favorable that the water
supply control means includes: water supply calculation means for
specifying the amount of water supply by the water supply means in order
to hold a predetermined ratio to the amount of oxidizing gas supply.
This structure allows the supply of water specified by the water supply
calculation means and thus realizes a constant ratio of the amount of
oxidizing gas supply by the oxidizing gas introducing means to the amount
of water supply by the water supply means. This structure accordingly
determines the degree of cooling the oxidizing unit in proportion to the
amount of heat generated by the oxidative reaction.
In accordance with still another preferable application, the apparatus of
the fundamental structure further includes: oxidative reaction detection
means for detecting a progress of an oxidative reaction in the oxidizing
unit; and control means for regulating an amount of water supply by the
water supply means according to the progress of the oxidative reaction.
In this preferable structure, the oxidative reaction detection means
detects the progress of the oxidative reaction in the oxidizing unit, and
the control means regulates the amount of water supply by the water supply
means according to the progress of the oxidative reaction. The oxidative
reaction in the oxidizing unit is exothermic, so that the enhanced
progress of the oxidative reaction increases the temperature of the
oxidizing unit. Variation in amount of water supply by the water supply
means changes the degree of cooling the oxidizing unit. Regulation of the
amount of water supply according to the progress of the oxidative reaction
thereby enables the degree of cooling the oxidizing unit to be controlled
according to the progress of the oxidative reaction.
The apparatus of this structure enables the oxidizing unit to be kept
within a desired temperature range, that is, the active temperature range
of the catalyst in the oxidizing unit, thereby effectively reducing the
concentration of carbon monoxide included in the hydrogen-rich gas.
In the above structure, the oxidative reaction detection means may include
means for measuring a temperature of the oxidizing unit, wherein the
oxidative reaction detection means detects said progress of the oxidative
reaction based on the temperature of the oxidizing unit. Since the
oxidative reaction in the oxidizing unit is exothermic, the observed
temperature of the oxidizing unit shows the progress of the oxidative
reaction. Regulation of the amount of water supply according to the
observed temperature of the oxidizing unit thus enables the temperature
control of the oxidizing unit according to the progress of the oxidative
reaction.
In the above structure, the oxidative reaction detection means may include
means for measuring a flow rate of the hydrogen-rich gas, wherein the
oxidative reaction detection means detects the progress of the oxidative
reaction based on the flow rate of the hydrogen-rich gas.
The progress of the oxidative reaction in the oxidizing unit is
significantly affected by the amount of carbon monoxide included in the
hydrogen-rich gas fed to the oxidizing unit. Since the amount of carbon
monoxide is varied in proportion to the total amount of the hydrogen-rich
gas, the flow rate of the hydrogen-rich gas shows the progress of the
oxidative reaction. Regulation of the amount of water supply according to
the observed flow rate of the hydrogen-rich gas thus enables the
temperature control of the oxidizing unit according to the progress of the
oxidative reaction.
In the above structure, the oxidative reaction detection means may include
means for measuring a concentration of carbon monoxide included in the
hydrogen-rich gas, wherein the oxidative reaction detection means detects
the progress of the oxidative reaction based on the concentration of
carbon monoxide. The progress of the oxidative reaction in the oxidizing
unit is affected by the concentration of carbon monoxide included in the
hydrogen-rich gas fed to the oxidizing unit. Regulation of the amount of
water supply according to the observed concentration of carbon monoxide in
the hydrogen-rich gas thus enables the temperature control of the
oxidizing unit according to the progress of the oxidative reaction.
The present invention is also directed to a fuel-cells generator system,
which includes: a reformer unit for converting a crude fuel including a
hydrocarbon as a primary component to a hydrogen-containing reformed gas;
oxidizing gas introducing means for introducing an oxygen-containing
oxidizing gas to the reformed gas; an oxidizing unit having a catalyst
that enables oxygen included in the oxidizing gas to be bonded to carbon
monoxide included in the reformed gas preferentially over hydrogen
included in the reformed gas; water supply means for feeding a supply of
water into the oxidizing unit; and a fuel cell for receiving a supply of
the reformed gas output from the oxidizing unit and generating an
electromotive force through an electrochemical reaction of the reformed
gas. This system is hereinafter referred to as the fuel-cells generator
system of the fundamental structure.
The reformer unit reforms a crude fuel including a hydrocarbon as the
primary component to a hydrogen-containing reformed gas. The oxidizing gas
introducing means introduces the oxidizing gas to the reformed gas, and
the catalyst in the oxidizing unit enables oxygen included in the
oxidizing gas to be bonded to carbon monoxide included in the reformed gas
preferentially over hydrogen. The water supply means feeds a supply of
water into the oxidizing unit. This structure exerts the same effects as
those of the apparatus of the fundamental structure discussed above and
reduces the concentration of carbon monoxide included in the reformed gas.
The fuel cell receives a supply of the reformed gas with the reduced
concentration of carbon monoxide and generates an electromotive force
through an electrochemical reaction of the reformed gas.
This structure significantly decreases the concentration of carbon monoxide
included in the reformed gas and effectively reduces the degree of
poisoning of a catalyst in the fuel cell with carbon monoxide. The fuel
cell is accordingly free from the lowered output due to the carbon
monoxide poisoning and is stably operated at high outputs.
In accordance with one preferable application, the fuel-cells generator
system of the fundamental structure further includes: water content
detection means for detecting a water content of an electrolyte membrane
in the fuel cell; and control means for regulating an amount of water
supply by the water supply means according to the water content of said
electrolyte membrane.
In the fuel cell, the operating condition affects the water content of the
electrolyte membrane. When the electrolyte membrane is too dried or too
wet, the output of the fuel cell is undesirably lowered. In this
preferable structure, the water content detection means detects the water
content of the electrolyte membrane, and the control means regulates the
amount of water supply according to the observed water content. This
structure enables the amount of water vapor in the reformed gas fed to the
fuel cell to be regulated according to the water content of the
electrolyte membrane.
The preferable structure maintains the water content of the electrolyte
membrane in the fuel cell within a predetermined range and prevents the
fuel cell from being too dried or too wet, thus ensuring stable high
outputs of the fuel cell.
In the fuel-cells generator system of the above structure, the water
content detection means may include means for measuring an electrical
resistance between electrodes in the fuel cell, wherein said water content
detection means detects said water content of said electrolyte membrane
based on said electrical resistance.
This structure measures the electrical resistance between the electrodes in
the fuel cell, in order to determine the water content of the electrolyte
membrane in the fuel cell. The amount of water supply by the water supply
means is then regulated according to the observed electrical resistance.
In accordance with another preferable application, the fuel-cells generator
system of the fundamental structure further includes: saturated amount of
water vapor calculation means for calculating a saturated amount of water
vapor included in the reformed gas that is output from the reformer unit
and flown into the fuel cell; and control means for regulating an amount
of water supply by the water supply means according to the saturated
amount of water vapor, in order to prevent a moisture of the reformed gas
from being supersaturated in the fuel cell.
In this preferable structure, the control means regulates the amount of
water supply according to the saturated amount of water vapor calculated
by the saturated amount of water vapor calculation means, thus preventing
the reformed gas from being flown into the fuel cell under a
supersaturated condition. This structure effectively prevents the
supersaturated water vapor in the reformed gas from aggregating to liquid
water in the fuel cell and accordingly blocking flow paths in the fuel
cell.
In accordance with still another preferable application, the fuel-cells
generator system of the fundamental structure further includes: reformed
gas pressure regulation means for regulating a pressure of the reformed
gas fed to the fuel cell; saturated amount of water vapor calculation
means for calculating a saturated amount of water vapor included in the
reformed gas that is output from the reformer unit and flown into the fuel
cell; and control means for controlling the reformed gas pressure
regulation means according to the saturated amount of water vapor, in
order to prevent a moisture of the reformed gas from being supersaturated
in the fuel cell.
In this structure, the control means controls the reformed gas pressure
regulation means according to the saturated amount of water vapor
calculated by the saturated amount of water vapor calculation means, thus
varying the pressure of the reformed gas and the saturated amount of water
vapor of the reformed gas. This prevents the reformed gas from being flown
into the fuel cell under a supersaturated condition. This structure
effectively prevents the supersaturated water vapor in the reformed gas
from aggregating to liquid water in the fuel cell and accordingly blocking
flow paths in the fuel cell.
It is preferable that the fuel-cells generator system of the above
structure further includes: oxidizing gas supply means for feeding an
oxygen-containing oxidizing gas to an oxygen electrode included in the
fuel cell; oxidizing gas pressure regulation means for regulating a
pressure of the oxidizing gas; and control means for controlling the
reformed gas pressure regulation means and the oxidizing gas pressure
regulation means, in order to enable a pressure difference between the
reformed gas and the oxidizing gas to be kept within a predetermined
range.
In the fuel-cells generator system of this structure, the control means
controls both the reformed gas pressure regulation means and the oxidizing
gas pressure regulation means, in order to keep the pressure difference
between the reformed gas and the oxidizing gas within a predetermined
range.
The electrolyte membrane in the fuel cell has an extremely small thickness
and may be damaged by the large pressure difference between the reformed
gas and the oxidizing gas. This structure, however, enables the pressure
difference between the reformed gas and the oxidizing gas to be kept
within the predetermined range, thus protecting the electrolyte membrane
from damages.
In accordance with another preferable application, the fuel-cells generator
system of the fundamental structure further includes stop-time control
means for actuating the water supply means to feed a supply of water to
the oxidizing unit at the time of stopping operation of the oxidizing
unit.
In this structure, the water supply means is actuated to feed a supply of
water to the oxidizing unit. This structure quickly lowers the temperature
of the oxidizing unit at the time of stopping operation of the oxidizing
unit, thereby rapidly stopping evolution of the hydrogen-rich gas.
In this preferable structure, the stop-time control means may include:
stop-time detection means for detecting a time of change from an operating
state to a ceased state of the fuel cell; and control means for actuating
the water supply means to feed a supply of water to the oxidizing unit at
the time of change to the ceased state.
At the time of change from the operating state to the ceased state of the
fuel cell, the water supply means is actuated to feed a supply of water to
the oxidizing unit. This structure quickly lowers the temperature of the
oxidizing unit at the time of change to the ceased state of the fuel cell,
thereby rapidly stopping evolution of the hydrogen-rich gas.
In accordance with still another preferable application, the fuel-cells
generator system of the fundamental structure further includes: water
recovery means for condensing moisture evolved from the fuel cell through
the electrochemical reaction and thereby recovering the moisture in the
form of liquid water; and water utilization means for utilizing the water
recovered by the water recovery means in the water supply means.
In the fuel cell, water vapor or water droplets are produced on the oxygen
electrode during power generation. In the fuel-cells generator system of
the above structure, the water recovery means condenses the moisture to
liquid water, and the water utilization means utilizes the recovered
water. This reduces the size of a water tank included in the water supply
means and decreases the required amount of water stored in the water tank.
Discharge of the remaining gas on the oxygen electrode to the atmosphere
causes white fumes. This structure, however, prevents such a phenomenon.
In accordance with another preferable application, the water supply means
may have water pressurizing means for pressurizing water and feeding said
pressurized water to said oxidizing unit by utilizing a flow of gaseous
exhaust from said fuel cell.
Pressurization of water is required, in order to feed water from the water
supply means to the oxidizing unit. When electrical energy generated by
the fuel cell is used as the power source of the pressurization, the
energy efficiency of the whole fuel-cells generator system is undesirably
lowered. This preferable structure, however, utilizes the flow of gaseous
exhaust to pressurize water, thus enabling water to be fed to the
oxidizing unit without lowering the energy efficiency of the whole
fuel-cells generator system.
The present invention is further directed to a method of reducing
concentration of carbon monoxide included in a hydrogen-rich gas, which
contains both hydrogen and carbon monoxide. The method includes the steps
of:
(a) introducing an oxygen-containing oxidizing gas to the hydrogen-rich
gas;
(b) activating a catalyst to enable oxygen included in the oxidizing gas to
be bonded to carbon monoxide included in the hydrogen-rich gas
preferentially over hydrogen included in the hydrogen-rich gas; and
(c) feeding a supply of water to the catalyst. This method is hereinafter
referred to as the method of the fundamental structure.
In the method of the fundamental structure, the oxidizing gas is introduced
into the hydrogen-rich gas containing carbon monoxide in the step (a). The
catalyst enables oxygen included in the oxidizing gas to oxidize carbon
monoxide included in the hydrogen-rich gas preferentially over hydrogen in
the step (b). Water is supplied to the catalyst in the step (c). Since the
oxidative reaction in the presence of the catalyst is exothermic, water
fed in the step (c) is vaporized on or in the vicinity of the catalyst.
This structure enables the catalyst to be directly cooled down by the heat
of vaporization.
The method of the fundamental structure directly cools down the catalyst.
This structure enhances the cooling efficiency and enables the catalyst to
be kept within an active temperature range, thereby sufficiently reducing
the concentration of carbon monoxide included in the hydrogen-rich gas.
In accordance with one preferable application, the method of the
fundamental structure further includes the steps of:
(d) measuring a temperature of the catalyst; and
(e) regulating an amount of water supply in the step (c), thereby enabling
the temperature of said catalyst to be kept within a predetermined range.
Regulation of the amount of water supply in the step (c) enables the
temperature of the catalyst measured in the step (d) to be kept within a
desired temperature range, that is, the active temperature range, thereby
effectively reducing the concentration of carbon monoxide included in the
hydrogen-rich gas.
In accordance with another preferable application, the method of the
fundamental structure further includes the steps of:
(f) measuring a concentration of carbon monoxide included in the
hydrogen-rich gas;
(g) regulating an amount of oxidizing gas supply in the step (a) according
to the concentration of carbon monoxide; and
(h) regulating an amount of water supply in the step (c) according to the
regulated amount of oxidizing gas supply.
In response to an increase in concentration of carbon monoxide included in
the hydrogen-rich gas, the amount of oxidizing gas supply is increased to
enhance the progress of the oxidative reaction in the step (f) and thereby
reduce the concentration of carbon monoxide included in the hydrogen-rich
gas. The enhanced progress of the oxidative reaction increases the heat
generated by the oxidative reaction. Regulation of the amount of water
supply according to the regulated amount of oxidizing gas supply enables
the degree of cooling the catalyst to be specified according to the amount
of heat generated by the oxidative reaction. This structure thus enables
the catalyst to be kept within a desired temperature range and effectively
reduces the concentration of carbon monoxide included in the hydrogen-rich
gas.
In accordance with still another preferable application, the method of the
fundamental structure further includes the steps of:
(i) detecting a progress of an oxidative reaction accelerated by the
catalyst; and
(j) regulating an amount of water supply in the step (c) according to the
progress of the oxidative.
In the step (j) of this method, the amount of water supply in the step (c)
is regulated according to the progress of the oxidative reaction detected
in the step (i). Since the oxidative reaction carried out in the presence
of the catalyst in the step (b) is exothermic, the enhanced progress of
the oxidative reaction increases the temperature of the catalyst. Varying
the amount of water supply in the step (c) changes the degree of cooling
the catalyst. Regulation of the amount of water supply according to the
progress of the oxidative reaction thereby controls the degree of cooling
the catalyst.
The method of this structure enables the catalyst to be kept within a
desired temperature range, that is, the active temperature range, thereby
effectively reducing the concentration of carbon monoxide included in the
hydrogen-rich gas.
These and other objects, features, aspects, and advantages of the present
invention will become more apparent from the following detailed
description of the preferred embodiments with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram schematically illustrating structure of a
fuel-cells generator system 10 including an apparatus for reducing the
concentration of carbon monoxide as a first embodiment according to the
present invention;
FIG. 2 illustrates a unit cell structure in a stack of fuel cells 20;
FIG. 3 shows inside of a selective CO oxidizing unit 34 with a connection
pipe 36 connected thereto;
FIG. 4 is a graph showing the performance of selective CO oxidizing
catalysts 50 with respect to a model gas;
FIG. 5 is a flowchart showing a CO concentration reduction routine executed
by the electronic control unit 90 of a reformer 30;
FIG. 6 shows inside of the selective CO oxidizing unit 34 with another
water supply means;
FIG. 7 shows inside of the selective CO oxidizing unit 34 with still
another water supply means;
FIG. 8 shows inside of the selective CO oxidizing unit 34 with another
water supply means;
FIG. 9 shows inside of the selective CO oxidizing unit 34 with still
another water supply means;
FIG. 10 is a block diagram schematically illustrating structure of another
fuel-cells generator system 210 as a second embodiment according to the
present invention;
FIG. 11 is a vertical sectional view illustrating structure of a carbon
monoxide sensor 212;
FIG. 12 is a flowchart showing a CO concentration reduction routine
executed by the electronic control unit 90A of the second embodiment;
FIG. 13 is a graph showing the relationship between the amount Qa of
oxidizing gas supply and the amount Qw of water supply;
FIG. 14 is a block diagram schematically illustrating structure of another
fuel-cells generator system 310 as a third embodiment according to the
present invention;
FIG. 15 is a flowchart showing a CO concentration reduction routine
executed by the electronic control unit 90B of the third embodiment;
FIG. 16 is a graph showing the relationship between the flow rate Qh of the
reformed gas and the target amount Qw of water supply;
FIG. 17 is a block diagram schematically illustrating structure of another
fuel-cells generator system 410 as a fourth embodiment according to the
present invention;
FIG. 18 is a flowchart showing a CO concentration reduction routine
executed by the electronic control unit 90C of the fourth embodiment;
FIG. 19 is a graph showing the relationship between the concentration D1 of
carbon monoxide and the target amount Qw of water supply;
FIG. 20 is a block diagram schematically illustrating structure of another
fuel-cells generator system 510 as a fifth embodiment according to the
present invention;
FIG. 21 is a flowchart showing a moistening control routine executed by the
electronic control unit 90D of the fifth embodiment;
FIG. 22 is a block diagram schematically illustrating structure of still
another fuel-cells generator system 610 as a sixth embodiment according to
the present invention;
FIG. 23 is a flowchart showing a maximum water supply calculation routine
executed by the electronic control unit 90E of the sixth embodiment;
FIG. 24 is a flowchart showing a CO concentration reduction routine
executed by the electronic control unit 90E of the sixth embodiment;
FIG. 25 is a block diagram schematically illustrating structure of another
fuel-cells generator system 710 as a seventh embodiment according to the
present invention;
FIG. 26 is a flowchart showing a fuel gas pressure control routine executed
by the electronic control unit 90F of the seventh embodiment;
FIG. 27 is a block diagram schematically illustrating structure of still
another fuel-cells generator system 810 as an eighth embodiment according
to the present invention;
FIG. 28 is a flowchart showing an oxidizing gas pressure control routine
executed by the electronic control unit 90G in the eighth embodiment;
FIG. 29 is a block diagram schematically illustrating structure of another
fuel-cells generator system 910 as a ninth embodiment according to the
present invention;
FIG. 30 is a flowchart showing a stop-time control routine executed by the
electronic control unit 90H in the ninth embodiment;
FIG. 31 is a block diagram schematically illustrating structure of still
another fuel-cells generator system 1010 as a tenth embodiment according
to the present invention; and
FIG. 32 is a block diagram schematically illustrating structure of another
fuel-cells generator system 1110 as an eleventh embodiment according to
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Some modes of carrying out the present invention are discussed below as
preferred embodiments. FIG. 1 is a block diagram schematically
illustrating structure of a fuel-cells generator system 10 including an
apparatus for reducing the concentration of carbon monoxide as a first
embodiment according to the present invention. The fuel-cells generator
system 10 includes a methanol tank 12 for storing methanol, a water tank
14 for storing water, a reformer 30 for receiving supplies of methanol and
water fed from the methanol tank 12 and the water tank 14 and producing a
hydrogen-containing gaseous fuel, and a stack of fuel cells 20. The stack
of fuel cells 20 includes polymer electrolyte fuel cells that receive
supplies of the gaseous fuel produced by the reformer 30 and an
oxygen-containing oxidizing gas, for example, the air, and generating
electricity.
The stack of fuel cells 20 consists of polymer electrolyte fuel cells as
mentioned above, and each unit cell has the structure shown in FIG. 2.
Each unit cell has an electrolyte membrane 21, an anode 22 and a cathode
23, which are gas diffusion electrodes arranged across the electrolyte
membrane 21 to construct a sandwich-like structure, separators 24 and 25,
which are disposed outside the sandwich-like structure and respectively
connected to the anode 22 and the cathode 23 to form flow paths 24p of
gaseous fuel and flow paths 25p of oxidizing gas, and collector plates 26
and 27, which are disposed further outside the separators 24 and 25 and
function as current collectors of the anode 22 and the cathode 23.
The electrolyte membrane 21 is an ion-exchange membrane composed of a
polymer material, such as a fluororesin, and shows favorable electrical
conductivity in the wet state. The anode 22 and the cathode 23 are made of
carbon paper, carbon sheet, or carbon cloth, wherein carbon powder with a
platinum catalyst carried thereon is incorporated in the interstices of
the carbon paper, carbon sheet, or carbon cloth.
The separators 24 and 25 are composed of a dense carbon plate. The
separator 24 has a plurality of ribs that are combined with the surface of
the anode 22 to define flow paths 24p of gaseous fuel, whereas the
separator 25 has a plurality of ribs that are combined with the surface of
the cathode 23 to define flow paths 25p of oxygen-containing gas. The
collector plates 26 and 27 are made of copper (Cu).
The stack of fuel cells 20 is obtained by stacking plural sets of the unit
cell structure, wherein the separator 24, the anode 22, the electrolyte
membrane 21, the cathode 23, and the separator 25 are arranged in this
sequence as shown in FIG. 2, and setting the collector plates 26 and 27
outside the stack of unit cell structures. Only the gas supply system on
the anode's side is illustrated in FIG. 1, while the gas supply system on
the cathode's side, the gas discharge system on the anode's side and the
gas discharge system on the cathode's side are omitted from the
illustration.
The reformer 30 includes a reformer unit 32 for receiving supplies of
methanol and water and generating a hydrogen-rich gas (reformed gas), a
selective CO oxidizing unit 34 for selectively oxidizing carbon monoxide
included in the reformed gas and thereby converting the reformed gas to a
hydrogen-rich gas containing a less amount of carbon monoxide (gaseous
fuel), a connection pipe 36 for feeding the reformed gas generated by the
reformer unit 32 to the selective CO oxidizing unit 34, a blower 38 for
feeding an oxygen-containing oxidizing gas, for example, the air, to the
connection pipe 36 via an induction pipe 37 connecting with the connection
pipe 36, a water inlet pipe 40 arranged down the Joint of the connection
pipe 36 with the induction pipe 37 for introducing water into the
connection pipe 36, and an electronic control unit 90 for controlling
operations of the respective elements of the reformer 30.
The reformer unit 32 receives supplies of methanol and water from the
methanol tank 12 and the water tank 14 and generates a reformed gas
containing hydrogen and carbon dioxide as shown in Equations (1) and (2)
given below (Equation (3) as a whole):
CH.sub.3 OH.fwdarw.CO+2H.sub.2 -21.7 kcal/mol (1)
CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 +9.8 kcal/mol (2)
CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +3H.sub.2 -11.9 kcal/mol (3)
In the actual state, the reactions of Equations (1) and (2) do not
completely proceed to the right. The reformed gas produced by the reformer
unit 32 includes small amounts of carbon monoxide as by-product and
non-reacted methanol. The concentration of carbon monoxide included in the
reformed gas depends upon the type of the catalyst packed in the reformer
unit 32, the operating temperature of the reformer unit 32, and the flow
rates of methanol and water supplied to the reformer unit 32 per unit
volume of the catalyst. Although not being illustrated, the reformer unit
32 is electrically connected to the electronic control unit 90. The
electronic control unit 90 regulates the supplies of methanol and water to
the reformer unit 32.
FIG. 3 shows the inside of the selective CO oxidizing unit 34 and the
connection pipe 36, which is linked with the selective CO oxidizing unit
34. The reformed gas produced by the reformer unit 32 and the oxidizing
gas fed from the blower 38 through the induction pipe 37 are led into the
selective CO oxidizing unit 34 via the connection pipe 36. The selective
CO oxidizing unit 34 is packed with selective CO oxidizing catalysts 50,
which include aluminum oxide with platinum carried on the surface thereof.
The selective CO oxidizing catalysts 50 enable carbon monoxide included in
the reformed gas to be oxidized preferentially over hydrogen, thereby
converting the reformed gas to a gaseous fuel having a low concentration
of carbon monoxide.
As shown in FIG. 3, the water inlet pipe 40 is inserted into the connection
pipe 36, and the selective CO oxidizing unit 34 receives a supply of water
from the water inlet pipe 40 via the connection pipe 36. The oxidative
reaction in the selective CO oxidizing unit 34 is exothermic and
accordingly increases the temperature in the selective CO oxidizing unit
34. Water fed from the water inlet pipe 40 through the connection pipe 36
is vaporized in the hot selective CO oxidizing unit 34. The heat of
vaporization then cools down the selective CO oxidizing unit 34. A
temperature sensor 52 is arranged to be in contact with the selective CO
oxidizing catalysts 50 in the selective CO oxidizing unit 34. The
temperature sensor 52 consists of thermocouples and measures the internal
temperature of the selective CO oxidizing unit 34. The temperature sensor
52 is electrically connected to the electronic control unit 90.
Referring back to FIG. 1, an electrically-operated valve 42 is disposed in
the middle of the water inlet pipe 40 and is electrically connected to the
electronic control unit 90. The electronic control unit 90 controls off
and on the electrically-operated valve 42, in order to start and stop a
supply of water to the selective CO oxidizing unit 34.
The selective CO oxidizing catalysts 50 packed in the selective CO
oxidizing unit 34 are obtained by making a platinum catalyst carried on
the surface of an aluminum oxide carrier. FIG. 4 is a graph showing the
performance of the selective CO oxidizing catalysts 50 against a model
gas.
The model gas used here was prepared by introducing water vapor to a
bottled gas, which had a predetermined composition of CO.sub.2 =25%,
CO=0.1%, and H.sub.2 =the residual percent, with a bubbler to the absolute
humidity of approximately 20%. A mixture of the model gas with an
oxidizing gas (the resulting molar ratio of oxygen to carbon monoxide
[O.sub.2 ]/[CO]=3) was flown into the catalysts at a predetermined flow
rate, which corresponded to the volume of approximately 5,000 times as
much as the total volume of the catalysts per hour on the dry gas basis.
Referring to the graph of FIG. 4, the selective CO oxidizing catalysts 50
lowered the concentration of carbon monoxide included in the reformed gas
after the catalytic reaction to or below a detection limit (that is, to or
below several ppm) at the reaction temperatures of 100.degree. C. to
140.degree. C.
Regulation of the operating temperature of the selective CO oxidizing unit
34 to the range of 100.degree. C. to 140.degree. C. (hereinafter referred
to as the optimum temperature range) significantly lowers the
concentration of carbon monoxide included in the gaseous fuel. In this
embodiment, the electronic control unit 90 opens or closes the
electrically-operated valve 42 based on the results of measurement of the
temperature sensor 52, thus enabling the operating temperature of the
selective CO oxidizing unit 34 to be kept within the optimum temperature
range.
Referring back to FIG. 1, the electronic control unit 90 is constructed as
a microcomputer-based logic circuit, and includes a CPU 92 for executing a
variety of logic and arithmetic operations according to preset control
programs, a ROM 94, in which control programs and control data required
for the variety of logic and arithmetic operations executed by the CPU 92
have been stored in advance, a RAM 96, which various data required for the
variety of logic and arithmetic operations executed by the CPU 92 are
temporarily written in and read from, and an input/output port 98 for
receiving detection signals from the temperature sensor 52 and the other
various sensors (not shown) and outputting driving signals, for example,
to the blower 38 and the electrically-operated valve 42, based on the
results of logic and arithmetic operations executed by the CPU 92.
The electronic control unit 90 of the fuel-cells generator system 10 thus
constructed carries out a control operation for decreasing the
concentration of carbon monoxide included in the gaseous fuel, based on a
routine of reducing CO concentration shown in the flowchart of FIG. 5. The
routine of FIG. 5 is repeatedly carried out at predetermined time
intervals, for example, at every 100 msec, after the reformer 30 has been
driven and reached a stationary state.
When the program enters the routine of FIG. 5, the CPU 92 first reads a
temperature T in the selective CO oxidizing unit 34 measured by the
temperature sensor 52 via the input/output port 98 at step S100. The
observed temperature T is compared with a lower limit T1 at step S110. The
lower limit T1, which corresponds to the lower limit of the optimum
temperature range of the selective CO oxidizing unit 34, is equal to
100.degree. C. and stored in the ROM 94 in this embodiment. In case that
the observed temperature T is lower than the lower limit T1 at step S110,
the program goes to step S120 to switch a position V of the
electrically-operated valve 42 disposed in the water inlet pipe 40 to a
closed state. This switching operation stops the supply of water through
the water inlet pipe 40 to the selective CO oxidizing unit 34, and thereby
increases the internal temperature of the selective CO oxidizing unit 34
that has not been cooled by the supplied water.
When the answer is negative at step S110, that is, in case that the
observed temperature is not lower than the lower limit T1, on the other
hand, the program proceeds to step S130, at which the observed temperature
T is further compared with an upper limit T2. The upper limit T2, which
corresponds to the upper limit of the optimum temperature range of the
selective CO oxidizing unit 34, is equal to 140.degree. C. and stored in
the ROM 94 in this embodiment. In case that the observed temperature T is
higher than the upper limit T2 at step S130, the program goes to step S140
to switch the position V of the electrically-operated valve 42 disposed in
the water inlet pipe 40 to an open state (either a full-open state or a
partially-open state with a predetermined opening). This switching
operation starts the supply of water through the water inlet pipe 40 to
the selective CO oxidizing unit 34, and thereby enables the internal
temperature of the selective CO oxidizing unit 34 to be decreased by the
heat of vaporization of the supplied water.
After the execution of either one of steps S120 and S140 or after the
negative answer at step S130, the program exits from this routine.
The CO concentration reduction routine controls on and off the
electrically-operated valve 42 according to the internal temperature of
the selective CO oxidizing unit 34 measured by the temperature sensor 52.
In case that the temperature T in the selective CO oxidizing unit 34 rises
too high, the supplied water cools down the selective CO oxidizing unit
34. In case that the temperature T is too low, on the contrary, the supply
of water for cooling is stopped. This procedure enables the operating
temperature of the selective CO oxidizing unit 34 to be kept within the
optimum temperature range of T1 to T2. Although the first embodiment does
not take into account a time lag between the supply of water to the
selective CO oxidizing unit 34 and an actual temperature change, one
preferable modification adds a predetermined value a to the lower limit T1
and subtracts a predetermined value .beta. from the upper limit T2, with a
view to taking into account the time lag.
As discussed above, in the fuel-cells generator system 10 of the first
embodiment, water is supplied through the water inlet pipe 40 to the
selective CO oxidizing unit 34 and the selective CO oxidizing unit 34 is
cooled down by the heat of vaporization of supplied water. The heat of
vaporization of supplied water directly cools down the selective CO
oxidizing catalysts 50 packed in the selective CO oxidizing unit 34. This
enhances the cooling efficiency and enables all the selective CO oxidizing
catalysts 50 packed in the selective CO oxidizing unit 34 to be maintained
within an active temperature range. The structure of the embodiment thus
sufficiently reduces the concentration of carbon monoxide included in the
gaseous fuel.
It is required to moisten the gaseous fuel fed to the stack of fuel cells
20. The structure of the first embodiment can decrease the required amount
of water added to the gaseous fuel for the purpose of moistening by the
amount of water fed to the selective CO oxidizing unit 34 for the purpose
of cooling. This structure uses water for the cooling purpose and does not
require any additional supply of energy for cooling down the selective CO
oxidizing unit 34. Whereas the conventional technique requires additional
energy for cooling down a cooling medium, which receives the heat from the
selective CO oxidizing unit 34 and, for example, passes through a
radiator, the fuel-cells generator system 10 of the embodiment does not
require such additional energy.
The fuel-cells generator system 10 of the first embodiment measures the
internal temperature of the selective CO oxidizing unit 34 with the
temperature sensor 52, and starts or stops the supply of water through the
water inlet pipe 40 based on the results of measurement by the temperature
sensor 52. This structure enables the operating temperature of the
selective CO oxidizing unit 34 to be kept within the active temperature
range of the selective CO oxidizing catalysts 50, thereby reducing the
concentration of carbon monoxide included in the gaseous fuel.
In the first embodiment, water is supplied from the water inlet pipe 40
through the connection pipe 36 to the selective CO oxidizing unit 34. As
shown in FIG. 6, one modified structure has a water inlet pipe 40A
inserted into the selective CO oxidizing unit 34. Water flowing through
the water inlet pipe 40A is thus directly sprinkled into the selective CO
oxidizing unit 34.
This modified structure preferably has a water injection valve 80 attached
to the free end of the water inlet pipe 40A for spraying water in a wide
angle as shown in FIG. 7. One available water injection valve is a spiral
injection valve utilizing revolutions of a liquid. The spiral injection
valve is a known device specified in, for example, `pp 192-201,
Kikaikogaku-kisokoza, Nenshokogaku, Kiso-to-oyo (Fundamental Lectures of
Mechanical Engineering, Combustion Engineering, Fundamentals and
Applications), Kiyoshi KOBAYASHI, Rikougaku Co., Ltd.` The spiral
injection valve realizes favorable atomization even under a low injection
pressure and enables the spray angle and the flow rate to be freely
designed. The spiral injection valve allows water to be atomized to the
particle diameter of 15 .mu.m, sprayed in the spray angle of 150 degrees,
and directly sprinkled onto a wide area of the selective CO oxidizing
catalysts 50. This structure quickly cools down a wide area of the
selective CO oxidizing catalysts 50, thereby homogeneously cooling down
the whole selective CO oxidizing unit 34.
Although the modified structure shown in FIG. 7 has only one water inlet
pipe 40A, another possible structure may have a plurality of water inlet
pipes. FIG. 8 shows the inside of the selective CO oxidizing unit 34 with
two water inlet pipes 40Aa and 40Ab. In this modified example, the
selective CO oxidizing catalysts 50 are divided into two groups 81 and 82
along the flow direction of the reformed gas in the selective CO oxidizing
unit 34, and the water inlet pipes 40Aa and 40Ab are respectively disposed
to sprinkle water onto the groups 81 and 82. Spiral injection valves 80a
and 80b are respectively attached to the ends of the water inlet pipes
40Aa and 40Ab.
Compared with the structure of FIG. 7, the structure of FIG. 8 can double
the area of the selective CO oxidizing catalysts 50, onto which water is
directly sprinkled, thereby more homogeneously cooling down the whole
selective CO oxidizing unit 34 and sufficiently reducing the concentration
of carbon monoxide included in the gaseous fuel.
Reduction of the concentration of carbon monoxide was evaluated in the
modified structure of FIG. 7 and the modified structure of FIG. 8. The
model gas used here was a hydrogen-rich gas having the concentration of
carbon monoxide equal to 0.6%. A mixture of the model gas with an
oxidizing gas (the resulting molar ratio of oxygen to carbon monoxide
[O.sub.2 ]/[CO]=3) was flown into the selective CO oxidizing catalysts 50,
and the flow rate of water was 1.36 mol/min.
In the structure of FIG. 7 with one water inlet pipe 40A, the concentration
of carbon monoxide included in the hydrogen-rich gas output from the
selective CO oxidizing unit 34 was 30 ppm. In the structure of FIG. 8 with
two water inlet pipes 40Aa and 40Ab, on the other hand, the concentration
of carbon monoxide included in the hydrogen-rich gas output from the
selective CO oxidizing unit 34 was 10 ppm. This measurement shows that the
structure with a plurality of water inlet pipes can reduce the
concentration of carbon monoxide included in the hydrogen-rich gas more
effectively than the structure with one water inlet pipe.
In the first embodiment and its modified structures, the supplies of
oxidizing gas and water are fed through separate pipes to the selective CO
oxidizing unit 34. As shown in FIG. 9, however, the supplies of oxidizing
gas and water may be fed together through an induction pipe 84 inserted
into the selective CO oxidizing unit 34. This structure enables water to
be sprayed by the oxidizing gas, so that the mechanism of spraying water
can be simplified.
In the first embodiment and its modified structures (except the structure
of FIG. 9), the water inlet pipe is arranged down the induction pipe of
the oxidizing gas. Another possible structure may, however, arrange the
water inlet pipe up the induction pipe of the oxidizing gas.
In the first embodiment discussed above, the selective CO oxidizing
catalysts 50 include an aluminum oxide carrier with the platinum catalyst
carried thereon. Other available carriers include silicon oxides,
zirconium oxides, cerium oxide, zinc oxide, calcium carbonate, copper
oxides, iron oxides, titanium oxides, cobalt oxides, and
yttria-partially-stabilized zirconia. Other available catalysts carried on
the carrier include rare metals, such as Pd, Ru, Rh, Ir, and Au, and
non-rare metals, such as Ni, Co, Cu, and Fe.
In the first embodiment discussed above, the supply of water through the
water inlet pipe 40 is started and stopped according to the observed
temperature T in the selective CO oxidizing unit 34. Another possible
structure regulates the amount of water supply through the water inlet
pipe 40 according to the observed temperature T. In case that the
temperature T in the selective CO oxidizing unit 34 rises too high, the
electrically-operated valve 42 is driven in the opening direction by a
predetermined amount. In case that the temperature T is too low, on the
contrary, the electrically-operated valve 42 is driven in the closing
direction by a predetermined amount. Still another possible structure
intermittently feeds water and regulates the interval of water supply,
thereby controlling the amount of water supply.
The following describes another fuel-cells generator system 210 as a second
embodiment according to the present invention. FIG. 10 is a block diagram
schematically illustrating structure of the fuel-cells generator system
210 of the second embodiment. The fuel-cells generator system 210 of the
second embodiment has a similar hardware structure to that of the
fuel-cells generator system 10 of the first embodiment, except that the
temperature sensor 52 is not set in the selective CO oxidizing unit 34 and
that a carbon monoxide sensor 212 for measuring the concentration of
carbon monoxide included in the gaseous fuel is disposed in the middle of
a flow path connecting the selective CO oxidizing unit 34 to the stack of
fuel cells 20. The same constituents are shown by like numerals and not
specifically described here.
The following describes structure of the carbon monoxide sensor 212, based
on the vertical sectional view of FIG. 11. The carbon monoxide sensor 212
includes an electrolyte membrane 220, two electrodes 222 and 224 arranged
across the electrolyte membrane 220 to constitute a sandwich-like
structure, two meshed metal plates 226 and 228 arranged across the
sandwich-like structure for preventing deflection of the sandwich-like
structure, two holders 230 and 232 for fixing the sandwich-like structure
and the metal plates 226 and 228, and an insulating member 234 for
coupling the holders 230 and 232 with each other in an electrically
insulating state.
The electrolyte membrane 220 is a proton-conductive membrane composed of a
polymer electrolyte material, such as a fluororesin. The electrodes 222
and 224 are made of an electrode base material, such as carbon paper,
carbon sheet, or carbon cloth, wherein carbon powder with a platinum
catalyst carried thereon is incorporated in the interstices of the
electrode base material.
The meshed metal plates 226 and 228 have the structure that enables gases
to be flown into the electrodes 222 and 224. Preferable material for the
meshed metal plates 226 and 228 has excellent electrical conductivity and
favorable rust-preventing properties and does not cause hydrogen
brittleness; for example, titanium and stainless steel.
The holders 230 and 232 respectively have flanges 230a and 232a projected
inward from the cylindrical holder structures 230 and 232. The electrolyte
membrane 220, the pair of electrodes 222 and 224, and the meshed metal
plates 226 and 228 are supported by these flanges 230a and 232a of the
holders 230 and 232. Preferable material for the holders 230 and 232 has
excellent electrical conductivity and favorable rust-preventing properties
and does not cause hydrogen brittleness; for example, titanium and
stainless steel. The holder 232 is provided with an O-ring 236, which
comes into contact with the electrolyte membrane 220 and prevents an
atmosphere of one electrode from leaking to the other electrode.
The holders 230 and 232 respectively have, on the circumference thereof,
outer screw threads 230b and 232b, which mate and engage with inner screw
threads 234a and 234b formed inside the insulating member 234. Engagement
of the mating screw threads 230b, 232b and 234a, 234b enables the holders
230 and 232 to connect with each other and securely support the sandwich
structure of electrode 222-electrolyte membrane 220-electrode 224 placed
therebetween. Preferable material for the insulating member 234 is, for
example, Teflon.
The carbon monoxide sensor 212 further includes a gas in-flow conduit 238
that is linked with one holder 230 via mating screw threads. The gas
in-flow conduit 238 leads a gaseous fuel, that is, an object gas to be
detected, into the electrode 222, and is composed of an insulating
material. The other holder 232 does not connect with any specific gas
conduit, but the electrode 234 is exposed to the atmosphere.
The carbon monoxide sensor 212 is also provided with an electric circuit
240, which electrically connects detection terminals 230T and 232T of the
holders 230 and 232 with each other. The electric circuit 240 includes a
voltmeter 242 and a resistor 244 for adjusting the load current, which are
arranged in parallel between the detection terminals 230T and 232T.
Connection of the voltmeter 242 is determined to give negative polarity to
the detection terminal 230T of the holder 230 on the side of the electrode
222 exposed to the gaseous fuel and positive polarity to the detection
terminal 232T of the holder 232 on the side of the electrode 224 exposed
to the atmosphere. Signals of the voltmeter 242 are output to an external
control system, that is, an electronic control unit 90A.
The carbon monoxide sensor 212 thus constructed is joined with the flow
path, which connects the selective CO oxidizing unit 34 to the stack of
fuel cells 20, via mating screw threads, and is used to measure the
concentration of carbon monoxide in the gaseous fuel fed to the stack of
fuel cells 20.
The following description regards the process of detecting carbon monoxide
included in the hydrogen-rich gas (that is, the gaseous fuel or the object
gas to be detected) with the carbon monoxide sensor 212. A supply of
gaseous hydrogen included in the hydrogen-rich gas is fed to the electrode
222 of the carbon monoxide sensor 212, while a supply of oxygen included
in the atmosphere is fed to the electrode 224. Reactions expressed by
Equations (4) and (5) given below accordingly proceed on the surface of
the electrodes 222 and 224 across the electrolyte membrane 220:
H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- (4)
2H.sup.+ +2e.sup.- +(1/2)O.sub.2 .fwdarw.H.sub.2 O (5)
These reactions are identical with those proceeding in the fuel cells,
which receive hydrogen and oxygen as fuels and generate electrical energy.
An electromotive force is thus generated between the electrodes 222 and
224. Since the resistor 244 is connected between the electrodes 222 and
224 in this embodiment, the voltmeter 242 measures the potential
difference between the electrodes 222 and 224 when a predetermined
electric current is flown through the circuit under a predetermined
loading connected between the electrodes 222 and 224. The potential
difference decreases with an increase in concentration of carbon monoxide
included in the object gas. This phenomenon is ascribed to the following
reasons.
The reaction expressed by Equation (4) given above proceeds on the
electrode 222, in which the carbon powder having the platinum catalyst
carried thereon is incorporated. Carbon monoxide existing in the object
gas is adsorbed by the catalyst and interferes with the catalytic action;
namely, carbon monoxide poisons the catalyst. The degree of poisoning is
large for the high concentration of carbon monoxide included in the object
gas and small for the low concentration of carbon monoxide. The potential
difference between the detection terminals 230T and 232T is measured,
while the reactions expressed by Equations (4) and (5) continuously
proceed on the electrodes 222 and 224. In this state, the potential
difference reflects the concentration of carbon monoxide included in the
object gas, and the measurement of potential difference determines the
concentration of carbon monoxide included in the object gas. Connection of
one detection terminal 230T with the other detection terminal 232T via the
resistor 244 enables the reactions of Equations (4) and (5) to
continuously proceed on the electrodes 222 and 224. Under such conditions,
the potential difference is measured between the detection terminals 230T
and 232T.
The relationship between the concentration of carbon monoxide and the
measurement of the voltmeter 242 is examined in advance with gases
containing known concentrations of carbon monoxide. The concentration of
carbon monoxide included in the object gas is then determined according to
this relationship. The existence of hydrogen does not affect the
sensitivity of detection in the measurement of the concentration of carbon
monoxide. The concentration of carbon monoxide included even in the
hydrogen-rich object gas, that is, the gaseous fuel supplied to the fuel
cells, can thus be determined with high accuracy.
The electronic control unit 90A of the fuel-cells generator system 210
carries out a control operation for decreasing the concentration of carbon
monoxide included in the gaseous fuel, which is different from that
carried out in the first embodiment but is based on a routine of reducing
CO concentration shown in the flowchart of FIG. 12. The routine of FIG. 12
is repeatedly carried out at predetermined time intervals, for example, at
every 100 msec, after a reformer 30A has been driven and reached a
stationary state.
When the program enters the routine of FIG. 12, a CPU 92A of the electronic
control unit 90A reads a concentration D of carbon monoxide included the
gaseous fuel at the outlet of the reformer 30A measured by the carbon
monoxide sensor 212 via an input/output port 98A at step S250. The
observed concentration D of carbon monoxide is compared with a
predetermined value D0 at step S260. The predetermined value D0 represents
an upper limit concentration of carbon monoxide that is allowable by the
stack of fuel cells 20. In case that the observed concentration D of
carbon monoxide is not greater than the predetermined value D0 at step
S260, the program does not require any specific control and exits from
this routine.
In case that the observed concentration D of carbon monoxide exceeds the
predetermined value D0 at step S260, on the other hand, the program
proceeds to step S270 to drive the blower 38 by a predetermined driving
amount S, in order to enable a predetermined amount Qa of the oxidizing
gas to be introduced into the selective CO oxidizing unit 34. A target
amount Qw of water supply introduced into the selective CO oxidizing unit
34 is then calculated at step S280 by dividing the predetermined amount Qa
of oxidizing gas that is specified at step S270 by a predetermined factor
k. At subsequent step S290, the CPU 92A regulates the position of the
electrically-operated valve 42 based on the target amount Qw of water
supply, thereby enabling water of the target amount Qw to be fed through
the water inlet pipe 40 to the selective CO oxidizing unit 34. Although
the electrically-operated valve 42 is switched between the open position
and the closed position in the first embodiment, the opening of the
electrically-operated valve 42 is regulated in the second embodiment.
After the execution of step S290, the program exits from this routine.
When the concentration D of carbon monoxide increases to exceed the
allowable level by the stack of fuel cells 20, the CO concentration
reduction routine of the second embodiment causes the oxidizing gas of the
predetermined amount Qa to be fed to the selective CO oxidizing unit 34
and water of the target amount Qw, which corresponds to the amount Qa of
oxidizing gas supply, to be introduced into the selective CO oxidizing
unit 34. The graph of FIG. 13 shows the amount Qw of water supply plotted
against the amount Qa of oxidizing gas supply. As clearly seen from the
graph of FIG. 13, the ratio of the amount Qa of oxidizing gas supply to
the amount Qw of water supply to the selective CO oxidizing unit 34 is
fixed to the factor k.
When the concentration of carbon monoxide included in the gaseous fuel
increases to a high level, the structure of the second embodiment
introduces the oxidizing gas into the selective CO oxidizing unit 34,
thereby reducing the concentration of carbon monoxide through the
oxidative reaction. Although the oxidative reaction is exothermic, the
heat of vaporization of water introduced into the selective CO oxidizing
unit 34 cools down the selective CO oxidizing unit 34. As discussed above,
the fixed ratio of the amount Qa of oxidizing gas supply to the amount Qw
of water supply determines the degree of cooling the selective CO
oxidizing unit 34 in response to the amount of heat generated by the
oxidative reaction.
This structure enables the operating temperature of the selective CO
oxidizing unit 34 to be kept within the desired temperature range, that
is, the active temperature range of the catalyst, thereby effectively
reducing the concentration of carbon monoxide included in the gaseous
fuel.
The structure of the second embodiment controls the amount Qw of water
supply to keep a fixed ratio to the amount Qa of oxidizing gas supply.
This simplifies the control procedure.
In the second embodiment, the predetermined amount Qa of the oxidizing gas
is introduced into the selective CO oxidizing unit 34 when the observed
concentration D of carbon monoxide rises to a high level. The amount Qa of
oxidizing gas supply may, however, be varied according to the difference
between the observed concentration D of carbon monoxide and the
predetermined value D0. Namely this modified structure increases the
amount Qa of oxidizing gas supply with an increase in difference. This
enables the concentration D of carbon monoxide to be quickly decreased to
or below the predetermined value D0. Even in the modified structure with
the varying amount Qa of oxidizing gas supply, since the amount Qw of
water supply is controlled to keep a fixed ratio to the amount Qa of
oxidizing gas supply, the operating temperature of the selective CO
oxidizing unit 34 is effectively kept within the desired temperature
range.
The second embodiment reduces the concentration of carbon monoxide by
introducing the predetermined amount Qa of the oxidizing gas to the
selective CO oxidizing unit 34 when the observed concentration D of carbon
monoxide rises to a high level. One modified structure continuously
introduces a fixed amount of the oxidizing gas into the selective CO
oxidizing unit 34 and increases the oxidizing gas supply by a
predetermined amount when the observed concentration D of carbon monoxide
rises to a high level. In this case, the amount of water supply is
controlled to keep a fixed ratio to the increased amount of oxidizing gas
supply. The structure of the second embodiment that introduces the
predetermined amount Qa of the oxidizing gas into the selective CO
oxidizing unit 34 in response to an increase in concentration D of carbon
monoxide to a high level may be generalized and applied to the structure
of the first embodiment.
The second embodiment determines the amount Qa of oxidizing gas supply to
the selective CO oxidizing unit 34, based on the data regarding the
concentration of carbon monoxide output from the carbon monoxide sensor
212. Another parameter may, however, be used for determination of the
amount Qa of oxidizing gas supply. The available parameter is, for
example, data regarding the battery output from the stack of fuel cells
20. When a decrease in battery output from the stack of fuel cells 20 is
detected, this modified structure ascribes the decrease in battery output
to CO poisoning of the catalyst on the anodes in the stack of fuel cells
20, and controls the amount Qa of oxidizing gas supply to the selective CO
oxidizing unit 34 in order to restore the battery output.
The following describes still another fuel-cells generator system 310 as a
third embodiment according to the present invention. FIG. 14 is a block
diagram schematically illustrating structure of the fuel-cells generator
system 310 of the third embodiment. The fuel-cells generator system 310 of
the third embodiment has a similar hardware structure to that of the
fuel-cells generator system 10 of the first embodiment, except that the
temperature sensor 52 is not set in the selective CO oxidizing unit 34 and
that the third embodiment has a gas flow meter 312 for measuring the flow
rate of the reformed gas output from the reformer unit 32 of a reformer
30B. The same constituents are shown by like numerals and not specifically
described here.
The gas flow meter 312 is disposed in the middle of the connection pipe 36
that connects the reformer unit 32 to the selective CO oxidizing unit 34
in the reformer 30B, and more particularly arranged up the joint of the
induction pipe 37 of the oxidizing gas with the connection pipe 36. The
gas flow meter 312 for measuring the flow rate of the reformed gas output
from the reformer unit 32 is electrically connected with an input/output
port 98B of an electronic control unit 90B.
The electronic control unit 90B of the fuel-cells generator system 310
carries out a control operation for decreasing the concentration of carbon
monoxide included in the gaseous fuel, which is different from that
carried out in the first embodiment but is based on a routine of reducing
CO concentration shown in the flowchart of FIG. 15. The routine of FIG. 15
is repeatedly carried out at predetermined time intervals, for example, at
every 100 msec, after the reformer 30B has been driven and reached a
stationary state.
When the program enters the routine of FIG. 15, a CPU 92B of the electronic
control unit 90B first reads a flow rate Qh of the reformed gas at the
outlet of the reformer unit 32 measured by the gas flow meter 312 via the
input/output port 98B at step S350. The CPU 92B then determines a target
amount Qw of water supply into the selective CO oxidizing unit 34 based on
the flow rate Qh of the reformed gas at step S360. In accordance with a
concrete procedure, the CPU 92B reads the target amount Qw of water supply
corresponding to the observed flow rate Qh of the reformed gas from a map,
which represents the relationship of FIG. 16 and is stored in advance in a
ROM 94B of the electronic control unit 90B. The CPU 92B subsequently
regulates the position of the electrically-operated valve 42 based on the
target amount Qw of water supply, thereby enabling water of the target
amount Qw to be flown through the water inlet pipe 40 and introduced into
the selective CO oxidizing unit 34 at step S370. After the execution of
step S370, the program exits from this control routine.
As discussed above, the CO concentration reduction routine of the third
embodiment enables water of the predetermined amount according to the
observed flow rate Qh of the reformed gas at the outlet of the reformer
unit 32 to be introduced into the selective CO oxidizing unit 34.
The heat release value in the selective CO oxidizing unit 34 is affected by
the amount of carbon monoxide included in the reformed gas fed to the
selective CO oxidizing unit 34. In case that the reformer unit 32 is
driven in a constant state, the amount of carbon monoxide included in the
reformed gas is varied in proportion to the total amount of the reformed
gas. Namely the increase in flow rate Qh of the reformed gas increases the
heat release value in the selective CO oxidizing unit 34. Regulation of
the amount Qw of water supply to the selective CO oxidizing unit 34 based
on the observed flow rate Qh of the reformed gas thus controls the degree
of cooling the selective CO oxidizing unit 34 in response to the heat
release value in the selective CO oxidizing unit 34. This structure
enables the operating temperature of the selective CO oxidizing unit 34 to
be kept within the desired temperature range.
The following describes still another fuel-cells generator system 410 as a
fourth embodiment according to the present invention. FIG. 17 is a block
diagram schematically illustrating structure of the fuel-cells generator
system 410 of the fourth embodiment. The fuel-cells generator system 410
of the fourth embodiment has a similar hardware structure to that of the
fuel-cells generator system 310 of the third embodiment, except that the
gas flow meter 312 is replaced by a carbon monoxide sensor 412 for
measuring the concentration of carbon monoxide included in the reformed
gas output from the reformer unit 32. The same constituents are shown by
like numerals and not specifically described here.
The carbon monoxide sensor 412 is identical with the carbon monoxide sensor
212 used in the second embodiment, and is disposed in the middle of the
connection pipe 36 that connects the reformer unit 32 to the selective CO
oxidizing unit 34 in a reformer 30C and more particularly arranged up the
joint of the induction pipe 37 of the oxidizing gas with the connection
pipe 36. The carbon monoxide sensor 412 is electrically connected to an
input/output port 98C of an electronic control unit 90C.
The electronic control unit 90C of the fuel-cells generator system 410
carries out a control operation for decreasing the concentration of carbon
monoxide included in the gaseous fuel, based on a routine of reducing CO
concentration shown in the flowchart of FIG. 18. The routine of FIG. 18 is
repeatedly carried out at predetermined time intervals, for example, at
every 100 msec, after the reformer 30C has been driven and reached a
stationary state.
When the program enters the routine of FIG. 18, a CPU 92C of the electronic
control unit 90C first reads a concentration D1 of carbon monoxide
included in the reformed gas at the outlet of the reformer unit 32
measured by the carbon monoxide sensor 412 via the input/output port 98C
at step S450. The CPU 92C then determines a target amount Qw of water
supply to the selective CO oxidizing unit 34 based on the observed
concentration D1 of carbon monoxide at step S460. In accordance with a
concrete procedure, the CPU 92C reads the target amount Qw of water supply
corresponding to the observed concentration D1 of carbon monoxide from a
map, which represents the relationship of FIG. 19 and is stored in advance
in a ROM 94C of the electronic control unit 90C. The CPU 92C subsequently
regulates the position of the electrically-operated valve 42 based on the
target amount Qw of water supply, thereby enabling water of the target
amount Qw to be flown through the water inlet pipe 40 and introduced into
the selective CO oxidizing unit 34 at step S470. After the execution of
step S470, the program exits from this control routine.
Like the first and the second embodiments, in the selective CO oxidizing
unit 34 of the fuel-cells generator system 410 of the fourth embodiment,
the amount of oxidizing gas supply is regulated according to the
concentration of carbon monoxide. While the second embodiment utilizes the
concentration D of carbon monoxide at the outlet of the selective CO
oxidizing unit 34 measured by the carbon monoxide sensor 212, the fourth
embodiment utilizes the concentration D1 of carbon monoxide at the inlet
of the selective CO oxidizing unit 34 measured by the carbon monoxide
sensor 412.
As discussed above, the CO concentration reduction routine of the fourth
embodiment regulates the amount Qw of water supply flowing through the
water inlet pipe 40 based on the observed concentration D1 of carbon
monoxide included in the reformed gas output from the reformer unit 32.
The amount of oxidizing gas supply flowing through the induction pipe 37
is also regulated according to the observed concentration D1 of carbon
monoxide.
The progress of the oxidative reaction in the selective CO oxidizing unit
34 is affected by the concentration D1 of carbon monoxide included in the
reformed gas fed to the selective CO oxidizing unit 34. Regulation of the
amount of water supply to the selective CO oxidizing unit 34 based on the
observed concentration D1 of carbon monoxide in the reformed gas thus
controls the degree of cooling the selective CO oxidizing unit 34 in
response to the progress of the oxidative reaction. The amount of
oxidizing gas supply is also regulated according to the observed
concentration D1 of carbon monoxide in this embodiment, so that a
sufficient amount of the oxidizing gas required for the oxidative reaction
is fed to the selective CO oxidizing unit 34. This results in a large heat
release value in the selective CO oxidizing unit 34 under the condition of
a high concentration D1 of carbon monoxide. The structure of the fourth
embodiment, however, controls the degree of cooling the selective CO
oxidizing unit 34 according to the observed concentration D1 of carbon
monoxide, thereby enabling the operating temperature of the selective CO
oxidizing unit 34 to be kept within the desired temperature range.
The following describes still another fuel-cells generator system 510 as a
fifth embodiment according to the present invention. FIG. 20 is a block
diagram schematically illustrating structure of the fuel-cells generator
system 510 of the fifth embodiment. The fuel-cells generator system 510 of
the fifth embodiment has an impedance meter 512 for measuring the
impedance of the stack of fuel cells 20, in addition to all the
constituents of the fuel-cells generator system 10 of the first embodiment
except the temperature sensor 52. The same constituents are shown by like
numerals and not specifically described here.
The impedance meter 512 is disposed between the anode 22 and the cathode 23
of a predetermined unit cell in the stack of fuel cells 20. The impedance
meter 512 applies an a.c. signal between the anode 22 and the cathode 23
and detects an a.c. resistance, that is, an impedance, from an observed
electric current in the case of application of a constant voltage and from
an observed voltage in the case of supply of a constant current. The
available a.c. signal is generally in the range of 100 Hz to 10 kHz. The
impedance meter 512 is electrically connected to an input/output port 98D
of an electronic control unit 90D.
The electronic control unit 90D of the fuel-cells generator system 510
carries out the CO concentration reduction control of the first embodiment
shown in the flowchart of FIG. 5 as well as a control operation for
moistening the stack of fuel cells 20. The moistening control is based on
a moistening control routine shown in the flowchart of FIG. 21. The
moistening control routine is repeatedly carried out at predetermined time
intervals, for example, at every 100 msec, independently of the CO
concentration reduction routine, after a reformer 30D has been driven and
reached a stationary state.
When the program enters the routine of FIG. 21, a CPU 92D of the electronic
control unit 90D first reads an impedance Z measured by the impedance
meter 512 via the input/output port 98D at step S550. The observed
impedance Z is then compared with a predetermined value Z0 at step S560.
Under certain driving conditions, the stack of fuel cells 20 fall into a
state in which the electrolyte membranes 21 are partly too wet or a state
in which the electrolyte membranes 21 are partly too dried. These states
can be detected by the impedance Z between the anode 22 and the cathode
23. In case that the observed impedance Z exceeds the predetermined value
Z0 at step S560, the program determines that the electrolyte membranes 21
are too dried in the stack of fuel cells 20 and proceeds to step S570 to
regulate the position V of the electrically-operated valve 42 disposed in
the water inlet pipe 40 in the opening direction by a predetermined amount
.DELTA.V. This operation increases the amount of water supplied through
the water inlet pipe 40 to the selective CO oxidizing unit 34, thereby
increasing the water vapor included in the gaseous fuel output from the
selective CO oxidizing unit 34. This results in moistening the stack of
fuel cells 20 and decreasing the impedance Z.
After the execution of step S570, the program exits from this control
routine. In case that the observed impedance Z is not greater than the
predetermined value Z0 at step S560, on the other hand, the program
immediately exits from this routine.
As discussed above, the fuel-cells generator system 510 of the fifth
embodiment determines whether or not the electrolyte membranes 21 are too
dried in the stack of fuel cells 20, based on the impedance between the
anode 22 and the cathode 23 measured by the impedance meter 512. When it
is determined that the electrolyte membranes 21 are too dried, the
structure of the fifth embodiment increases the amount of water supplied
through the water inlet pipe 40 to the selective CO oxidizing unit 34,
thereby increasing the amount of water vapor included in the gaseous fuel
fed from the selective CO oxidizing unit 34 to the stack of fuel cells 20.
This operation enables the water content of the electrolyte membranes 21
in the stack of fuel cells 20 to be maintained within a predetermined
range. This accordingly prevents the electrolyte membranes 21 in the stack
of fuel cells 20 from being too dried or too wet, thus ensuring stable
high outputs from the stack of fuel cells 20.
In the fifth embodiment, the impedance meter 512 measures the impedance
between the anode 22 and the cathode 23 of a predetermined unit cell in
the stack of fuel cells 20. Another possible structure may, however,
measure the impedance for all the unit cells constituting the stack of
fuel cells 20 and calculate a mean impedance or sum up all the observed
impedances. The structural characteristic of the stack of fuel cells 20
teaches, for example, that the unit cells close to the end plates tend to
be too wet or too dried. The impedance may accordingly be measured for
these specific cells.
The structure of measuring the impedance may be replaced by the structure
of measuring a d.c. resistance between electrodes of a predetermined unit
cell in the stack of fuel cells 20. Since the fuel cells generate a d.c.
electromotive force during operation, it is generally impossible to
measure the d.c. resistance directly. There is, however, an available
method of directly measuring the d.c. resistance. This method cuts off a
loading connected to the stack of fuel cells 20 for a very short time
period, for example, several milliseconds, measures the d.c. resistance in
the cut-off state, and again connects the loading to the stack of fuel
cells 20. The available method then calculates the water content of the
electrolyte membrane 21 in the stack of fuel cells 20 from the observed
d.c. resistance and regulates the amount of water supply to the selective
CO oxidizing unit 34 based on the result of calculation.
The following describes still another fuel-cells generator system 610 as a
sixth embodiment according to the present invention. FIG. 22 is a block
diagram schematically illustrating structure of the fuel-cells generator
system 610 of the sixth embodiment. The fuel-cells generator system 610 of
the sixth embodiment has a flow sensor 612 for measuring a flow rate Q of
the gaseous fuel fed to the stack of fuel cells 20 and a pressure sensor
614 for measuring a pressure P of the gaseous fuel, in addition to all the
constituents of the fuel-cells generator system 310 of the third
embodiment. The same constituents are shown by like numerals and not
specifically described here.
The flow sensor 612 and the pressure sensor 614 are respectively disposed
in the middle of a flow path that connects a reformer 30E with the stack
of fuel cells 20 and measure the flow rate Q and the pressure P of the
gaseous fuel supplied to the stack of fuel cells 20. Both the flow sensor
612 and the pressure sensor 614 are electrically connected to an
input/output port 98E of an electronic control unit 90E.
The electronic control unit 90E of the fuel-cells generator system 610
carries out a CO concentration reduction control, which is similar to that
executed in the third embodiment, as well as a process of determining a
maximum amount of water supply to the selective CO oxidizing unit 34. This
determination is based on a maximum water supply calculation routine shown
in the flowchart of FIG. 23. The routine of FIG. 23 is repeatedly carried
out at predetermined time intervals, for example, at every 100 msec,
independently of the CO concentration reduction routine, after the
reformer 30E has been driven and reached a stationary state.
When the program enters the routine of FIG. 23, a CPU 92E of the electronic
control unit 90E first reads the flow rate Q and the pressure P of the
gaseous fuel measured by the flow sensor 612 and the pressure sensor 614
via the input/output port 98E at step S630. The CPU 92E then determines a
partial pressure A of water vapor in the gaseous fuel flown into the stack
of fuel cells 20 based on the input pressure P of the gaseous fuel and an
operating temperature T of the stack of fuel cells 20 at step S640.
Although not being discussed specifically, the stack of fuel cells 20 is
controlled to keep a substantially constant operating temperature T, for
example, 80.degree. C. in polymer electrolyte fuel cells. This value
80.degree. C. is used for the determination at step S640. In accordance
with a concrete procedure, at step S640, the CPU 92E reads the partial
pressure A of water vapor corresponding to the gas pressure P and the
operating temperature T from a map that has been stored previously in a
ROM 94E. At subsequently step S650, the CPU 92E calculates a saturated
amount B of water vapor in the gaseous fuel flown into the stack of fuel
cells 20 by multiplying the partial pressure A of water vapor by the gas
flow rate Q read at step S630.
The CPU 92E then calculates a ratio of the hydrocarbon to water (water
vapor) (hereinafter referred to as the S/C ratio) subjected to the
reforming reaction in the reformer unit 32 based on the regulation of
supplies of methanol and water to the reformer unit 32 according to
another control routine, and calculates an amount W of water per unit
volume from the S/C ratio at step S660. The program then proceeds to step
S670 to calculate a humidity H by dividing the amount W of water
calculated at step S660 by the partial pressure A of water vapor obtained
at step S640. The CPU 92E finally subtracts the product of the humidity H
calculated at step S670 and the gas flow rate Q read at step S630 from the
saturated amount B of water vapor obtained at step S650, thereby
determining a maximum amount Wm of water supply at step S680. After the
execution of step S680, the program exits from this routine.
The maximum amount Wm of water supply thus obtained enables the water
content included in the gaseous fuel fed from the reformer 30E to the
stack of fuel cells 20 to be saturated at the pressure P of the gaseous
fuel and the operating temperature T of the fuel cells, that is, the water
supply attaining the humidity of 100%.
The electronic control unit 90E also carries out a control operation for
decreasing the concentration of carbon monoxide based on a CO
concentration reduction routine shown in the flowchart of FIG. 24. Like
the CO concentration reduction routine of the third embodiment shown in
the flowchart of FIG. 15, this routine is repeatedly executed at
predetermined time intervals, for example, at every 100 msec.
When the program enters the routine of FIG. 24, the CPU 92E of the
electronic control unit 90E first carries out the processing of steps S690
and S692, which are identical with steps S350 and S360 in the CO
concentration reduction routine of the third embodiment, in order to
determine the target amount Qw of water supply based on the flow rate Qh
of the reformed gas. The target amount Qw of water supply is then compared
with the maximum amount Wm of water supply obtained in the maximum water
supply calculation routine discussed above at step S694. In case that the
target amount Qw of water supply exceeds the maximum amount Wm of water
supply, the target amount Qw of water supply is restricted to the maximum
amount Wm of water supply at step S696. In case that the target amount Qw
of water supply does not exceed the maximum amount Wm of water supply at
step S694, on the other hand, the program skips the processing of step
S696 and keeps the target amount Qw of water supply calculated at step
S692 unchanged.
The program then proceeds to step S698, which is identical with step S370
in the CO concentration reduction routine of the third embodiment, to
regulate the position of the electrically-operated valve 42 based on the
target amount Qw of water supply. After the execution of step S698, the
program exits from this routine.
As discussed above, the CO concentration reduction routine of the sixth
embodiment prevents the target amount Qw of water supply fed to the
selective CO oxidizing unit 34 from exceeding the maximum amount Wm of
water supply, thereby preventing the water content in the gaseous fuel
from being supersaturated at the pressure P of the gaseous fuel and the
operating temperature T of the fuel cells.
In the fuel-cells generator system 610 of the sixth embodiment, the gaseous
fuel output from the reformer 30E is not flown into the stack of fuel
cells 20 under the supersaturated condition. This structure effectively
prevents the supersaturated water vapor in the gaseous fuel from
aggregating to liquid water in the stack of fuel cells 20 and accordingly
blocking the flow paths 24p of gaseous fuel in the fuel cells. This
feature enables the stack of fuel cells 20 to be stably and continuously
driven at high battery outputs.
The fuel-cells generator system 610 of the sixth embodiment adds the
restriction by the maximum amount Wm of water supply to the fuel-cells
generator system 310 of the third embodiment. The restriction by the
maximum amount Wm of water supply may, however, be added to the fuel-cells
generator systems of the other embodiments (that is, the first, the
second, the fourth, and the fifth embodiments). In the second, the fourth,
and the fifth embodiments, the processing of steps S694 and S696 discussed
above is carried out. Namely when the target amount Qw of water supply
obtained in the CO concentration reduction routine exceeds the maximum
amount Wm of water supply, the target amount Qw of water supply is
restricted to the maximum amount Wm of water supply. In the first
embodiment, on the other hand, at step S140 in the CO concentration
reduction routine, the position of the electrically-operated valve 42 is
regulated to a specific value that restricts the actual amount of water
supply to the maximum amount Wm of water supply.
Like the sixth embodiment, these structures prevent the gaseous fuel output
from the reformer from being flown into the stack of fuel cells 20 under
the supersaturated condition. This effectively prevents the supersaturated
water vapor in the gaseous fuel from aggregating to liquid water in the
stack of fuel cells 20 and accordingly blocking the flow paths 24p of
gaseous fuel in the fuel cells.
The following describes another fuel-cells generator system 710 as a
seventh embodiment according to the present invention. FIG. 25 is a block
diagram schematically illustrating structure of the fuel-cells generator
system 710 of the seventh embodiment. The fuel-cells generator system 710
of the seventh embodiment has a back-pressure regulating valve 714 for
regulating an opening of a gaseous fuel discharge conduit 712 that
discharges the gaseous fuel from the stack of fuel cells 20 to the
outside, in addition to all the constituents of the fuel-cells generator
system 610 of the sixth embodiment. The same constituents are shown by
like numerals and not specifically described here.
The back-pressure regulating valve 714 is electrically connected to an
input/output port 98F of an electronic control unit 90F. The position of
the back-pressure regulating valve 714 is regulated by control signals
output from the electronic control unit 90F.
The electronic control unit 90F of the fuel-cells generator system 710
carries out a control operation for regulating the pressure of the gaseous
fuel, as well as the maximum water supply calculation routine of the sixth
embodiment shown in the flowchart of FIG. 23 and the CO concentration
reduction routine of the third embodiment shown in the flowchart of FIG.
15. Regulation of the pressure of the gaseous fuel is based on a fuel gas
pressure control routine shown in the flowchart of FIG. 26. This routine
is repeatedly carried out at predetermined time intervals, for example, at
every 100 msec, independently of the CO concentration reduction routine
and the maximum water supply calculation routine, after a reformer 30F has
been driven and reached a stationary state.
When the program enters the routine of FIG. 26, a CPU 92F of the electronic
control unit 90F first reads the maximum amount Wm of water supply
obtained in the maximum water supply calculation routine discussed in the
sixth embodiment and the target amount Qw of water supply obtained in the
CO concentration reduction routine discussed in the third embodiment at
step S750. The target amount Qw of water supply is then compared with the
maximum amount Wm of water supply at step S760. When the target amount Qw
of water supply exceeds the maximum amount Wm of water supply, the program
proceeds to step S770 to drive the back-pressure regulating valve 714 in
the closing direction by a predetermined small amount .DELTA.V, thereby
increasing the gas pressure P in the gaseous fuel discharge conduit 712.
After the execution of step S770, the program exits from this routine.
When it is determined that the target amount Qw of water supply does not
exceed the maximum amount Wm of water supply at step S760, on the
contrary, the program skips the processing of step S770 and exits from
this routine.
As discussed above, when the target amount Qw of water supply fed to the
selective CO oxidizing unit 34 exceeds the maximum amount Wm of water
supply, the fuel gas pressure control routine drives the back-pressure
regulating valve 714 in the closing direction, so as to gradually enhance
the gas pressure P in the gaseous fuel discharge conduit 712. The increase
in pressure P of the gaseous fuel increases the saturated amount of water
vapor in the gaseous fuel. This structure accordingly prevents the gaseous
fuel from being flown into the fuel cells under the supersaturated
condition, without changing the target amount Qw of water supply
calculated in the CO concentration reduction routine.
Like the sixth embodiment, the fuel-cells generator system 710 of the
seventh embodiment effectively prevents the supersaturated water vapor in
the gaseous fuel from aggregating to liquid water in the stack of fuel
cells 20 and accordingly blocking the flow paths 24p of gaseous fuel in
the fuel cells. This feature enables the stack of fuel cells 20 to be
stably and continuously driven at high battery outputs. The structure of
the seventh embodiment enables water of the target amount Qw calculated in
the CO concentration reduction routine to be fed into the selective CO
oxidizing unit 34. Namely water of greater than a required amount can be
fed into the selective CO oxidizing unit 34.
Like the fuel-cells generator system 610 of the sixth embodiment, the
additional structure of the seventh embodiment is applied to the
fuel-cells generator system 310 of the third embodiment. This additional
structure may, however, be applied to the other embodiments (that is, the
first, the second, the fourth, and the fifth embodiments).
The following describes still another fuel-cells generator system 810 as an
eighth embodiment according to the present invention. FIG. 27 is a block
diagram schematically illustrating structure of the fuel-cells generator
system 810 of the eighth embodiment. The fuel-cells generator system 810
of the eighth embodiment has a back-pressure regulating valve 814 for
regulating an opening of an oxidizing gas discharge conduit 812 that
discharges the oxidizing gas from the stack of fuel cells 20 to the
outside, a first pressure sensor 816 disposed in the upstream of the
gaseous fuel discharge conduit 712 for measuring the pressure of the
gaseous fuel, and a second pressure sensor 818 disposed in the upstream of
the oxidizing gas discharge conduit 812 for measuring the pressure of the
oxidizing gas, in addition to all the constituents of the fuel-cells
generator system 710 of the seventh embodiment. The same constituents are
shown by like numerals and not specifically described here.
The back-pressure regulating valve 814 is electrically connected to an
input/output port 98G of an electronic control unit 90G. The position of
the back-pressure regulating valve 814 is regulated by control signals
output from the electronic control unit 90G. The first pressure sensor 816
and the second pressure sensor 818 are also electrically connected to the
input/output port 98G of the electronic control unit 90G and output the
observed pressures to the electronic control unit 90G.
The electronic control unit 90G of the fuel-cells generator system 810
carries out a control operation for regulating the pressure of the
oxidizing gas, as well as the routines of the seventh embodiment (that is,
the maximum water supply calculation routine, the CO concentration
reduction routine, and the fuel gas pressure control routine). Regulation
of the pressure of the oxidizing gas is based on an oxidizing gas pressure
control routine shown in the flowchart of FIG. 28. This routine is
repeatedly carried out at predetermined time intervals, for example, at
every 100 msec, independently of the other routines, after a reformer 30G
has been driven and reached a stationary state.
When the program enters the routine of FIG. 28, a CPU 92G of the electronic
control unit 90G first reads a pressure Pa of the gaseous fuel and a
pressure Pc of the oxidizing gas respectively measured by the first and
the second pressure sensors 816 and 818 at step S850.
A pressure difference .DELTA.P is then calculated by subtracting the
pressure Pc of the oxidizing gas from the pressure Pa of the gaseous fuel
at step S860. The program subsequently determines whether or not the
pressure difference .DELTA.P is greater than zero at step S870 and
determines whether or not the pressure difference .DELTA.P is not greater
than a predetermined value .alpha. (>0) at step S880. The predetermined
value .alpha. strongly depends upon the properties of the electrolyte
membrane, especially the thickness thereof, and is, for example, equal to
0.2 [kPa]. In case of the negative answer at step S870, that is, when the
pressure difference .DELTA.P is determined to be not greater than zero,
the program proceeds to step S890 to regulate the position of the
back-pressure regulating valve 814 in the oxidizing gas discharge conduit
812 in the opening direction by a predetermined amount V0, so as to
decrease the pressure Pc of the oxidizing gas. This causes the pressure
difference .DELTA.P to become greater than zero.
In case of the negative answer at step S880, that is, when the pressure
difference .DELTA.P is greater than the predetermined value .alpha., the
program proceeds to step S892 to regulate the position of the
back-pressure regulating valve 814 in the oxidizing gas discharge conduit
812 in the closing direction by the predetermined amount V0, so as to
increase the pressure Pc of the oxidizing gas. This causes the pressure
difference .DELTA.P to become equal to or less than the predetermined
value .alpha..
After the execution of either one of steps S890 and S892 or in case of the
affirmative answers at both steps S870 and S880, that is, when the
relationship of 0<.DELTA.P.ltoreq..alpha. is satisfied, the program goes
to RETURN and exits from this routine.
As discussed above, the oxidizing gas pressure control routine regulates
the position of the back-pressure regulating valve 814 in the oxidizing
gas discharge conduit 812, thereby enabling the pressure difference
.DELTA.P obtained by subtracting the pressure Pc of the oxidizing gas from
the pressure Pa of the gaseous fuel to be kept within the range of zero to
the predetermined value .alpha.. Even when the position of the
back-pressure regulating valve 714 in the gaseous fuel discharge conduit
712 is regulated in the closing direction to increase the pressure of the
gaseous fuel in the fuel gas pressure control routine discussed in the
seventh embodiment, the structure of the eighth embodiment prevents the
pressure difference .DELTA.P between the pressure Pa of the gaseous fuel
and the pressure Pc of the oxidizing gas from exceeding a predetermined
range.
The fuel-cells generator system 810 of the eighth embodiment exerts the
same effects as those of the seventh embodiment and further enables the
pressure difference .DELTA.P between the pressure Pa of the gaseous fuel
and the pressure Pc of the oxidizing gas to be kept within a predetermined
range. This effectively prevents the electrolyte membranes 21 in the stack
of fuel cells 20 from being damaged by the pressure difference .DELTA.P.
The following describes still another fuel-cells generator system 910 as a
ninth embodiment according to the present invention. FIG. 29 is a block
diagram schematically illustrating structure of the fuel-cells generator
system 910 of the ninth embodiment. The fuel-cells generator system 910 of
the ninth embodiment has a fuel cells-operation electronic control unit
920 for controlling operation of the stack of fuel, cells 20, in addition
to all the constituents of the fuel-cells generator system 10 of the first
embodiment. The same constituents are shown by like numerals and not
specifically described here.
Like an electronic control unit 90H of a reformer 30H, the fuel-cells
operation electronic control unit 920 includes a CPU 922, a ROM 924, a RAM
926, and an input/output port 928 for controlling operation of the stack
of fuel cells 20. The electronic control unit 90H is electrically
connected to the fuel-cells operation electronic control unit 920 and thus
receives information on the operating state of the stack of fuel cells 20.
The electronic control unit 90H of the reformer 30H carries out a control
operation at the time of stopping fuel cells, as well as the CO
concentration reduction routine of the first embodiment shown in the
flowchart of FIG. 5. The control operation at the time of stopping fuel
cells is based on a stop-time control routine shown in the flowchart of
FIG. 30. This routine is repeatedly carried out at predetermined time
intervals, for example, at every 100 msec, independently of the CO
concentration reduction routine, after the reformer 30H has been driven
and reached a stationary state.
When the program enters the routine of FIG. 30, a CPU 92H of the electronic
control unit 90H first determines whether or not a stop signal has been
output from the fuel-cells operation electronic control unit 920 at step
S950. The fuel-cells operation electronic control unit 920 outputs a stop
signal when the stack of fuel cells 20 has been changed from the operating
state to the ceased state. When it is determined at step S950 that the
stop signal has been output, the program proceeds to step S960 to drive
the electrically-operated valve 42 in the water inlet pipe 40 to the
full-open position. This operation enables a large amount of water to be
fed from the water inlet pipe 40 to the selective CO oxidizing unit 34,
thereby abruptly decreasing the internal temperature of the selective CO
oxidizing unit 34 by the heat of vaporization of water.
The structure of the ninth embodiment thus quickly decreases the
temperature of the selective CO oxidizing unit 34 at the time of stopping
the stack of fuel cells 20. This results in rapidly stopping output of the
gaseous fuel.
The stop signal may be output only in the case of a normal stop of the
stack of fuel cells 20, only in the case of an emergency stop of the stack
of fuel cells 20, or in both the cases.
Although the stop signal is output at the time of stopping the stack of
fuel cells 20 in the ninth embodiment, the stop signal may be output at
the time of stopping the whole fuel-cells generator system including the
stack of fuel cells 20. This structure is also applicable to a control
operation of regulating the electrically-operated valve 42 in the water
inlet pipe 40 to the full-open position, prior to a stop of the reformer
30.
The following describes still another fuel-cells generator system 1010 as a
tenth embodiment according to the present invention. FIG. 31 is a block
diagram schematically illustrating structure of the fuel-cells generator
system 1010 of the tenth embodiment. The fuel-cells generator system 1010
of the tenth embodiment has a condenser 1030 disposed in an oxidizing gas
discharge conduit 1020 that discharges the oxidizing gas from the stack of
fuel cells 20 to the outside and a water conduit 1040 that connects the
condenser 1030 with the water tank 14, in addition to all the constituents
of the fuel-cells generator system 10 of the first embodiment. The same
constituents are shown by like numerals and not specifically described
here.
The condenser 1030 condenses the water vapor to produce water. The water
vapor evolved at the cathodes in the stack of fuel cells 20 during power
generation is recovered as water. The output of the condenser 1030 is
connected to the water tank 14, from which water is fed to the reformer
unit 32 and the selective CO oxidizing unit 34, via the water conduit
1040. Water produced by the condenser 1030 is accordingly sent to the
water tank 14. This structure enables water used for the pyrolysis and
water fed to the selective CO oxidizing unit 34 to be successively
supplied through operation of the stack of fuel cells 20. This favorably
reduces the size of the water tank 14 and lessens the amount of water
stored in the water tank 14. Discharge of the remaining gas evolved at the
cathodes to the atmosphere causes white fumes. This structure, however,
prevents such a phenomenon.
In the stack of polymer electrolyte fuel cells, the gaseous fuel is
generally moistened for the purpose of moistening the electrolyte
membranes. Water is also fed through the water inlet pipe 40 to the
selective CO oxidizing unit 34. This causes the gas discharged from the
anodes in the stack of fuel cells 20 to include a large volume of water
vapor or water droplets. The condenser 1030 may accordingly be disposed in
a gaseous fuel discharge conduit 1050 that discharges the gaseous fuel
from the stack of fuel cells 20 to the outside, instead of the oxidizing
gas discharge conduit 1020, or alternatively be disposed both in the
oxidizing gas discharge conduit 1020 and the gaseous fuel discharge
conduit 1050.
The following describes still another fuel-cells generator system 1110 as
an eleventh embodiment according to the present invention. FIG. 32 is a
block diagram schematically illustrating structure of the fuel-cells
generator system 1110 of the eleventh embodiment. The fuel-cells generator
system 1110 of the eleventh embodiment has a water pressurizing mechanism
1120 in the middle of the water inlet pipe 40, in addition to all the
constituents of the fuel-cells generator system 10 of the first
embodiment. The same constituents are shown by like numerals and not
specifically described here.
The water pressurizing mechanism 1120 includes a turbine compressor 1122
disposed in the middle of an oxidizing gas discharge conduit 1112 that
discharges the oxidizing gas from the stack of fuel cells 20 to the
outside, and a turbine 1124 disposed in the middle of the water inlet pipe
40 and linked coaxially with the turbine compressor 1122. The water
pressurizing mechanism 1120 utilizes the energy of the flow of gaseous
exhaust from the stack of fuel cells 20 to pressurize water flowing in the
water inlet pipe 40. Although not being specifically mentioned in the
above embodiments, pressurizing means, such as a pump, should be used to
pressurize water, in order to supply water from the water inlet pipe 40 to
the selective CO oxidizing unit 34. When electrical energy generated by
the stack of fuel cells 20 is used as the power source of the pressurizing
means, the energy efficiency of the whole fuel-cells generator system is
undesirably lowered. The structure of the eleventh embodiment, however,
utilizes the flow of gaseous exhaust to pressurize water. This enables
water to be fed to the selective CO oxidizing unit 34 without lowering the
energy efficiency of the whole fuel-cells generator system.
In this embodiment, the turbine compressor 1122 is driven by the oxidizing
gas discharged from the anodes in the stack of fuel cells 20. One modified
structure may, however, drive the turbine compressor 1122 by the gaseous
fuel discharged from the cathodes in the stack of fuel cells 20.
Although all the above embodiments are applied to the polymer electrolyte
fuel cells (PEFC), the principle of the present invention is also
applicable to other fuel-cells generator systems including phosphate fuel
cells (PAFC), direct methanol fuel cells (DMFC), alkali fuel cells (AFC),
molten carbonate fuel cells (MCFC), and solid oxide fuel cells (SOFC).
In all the embodiments discussed above, the methanol reformer is used as
the supply source of the hydrogen-rich gas. The principle of the present
invention may, however, be applicable to other fuel-cells generator
systems in combination with other reformers that produce a reformed gas
containing hydrogen as a primary component. Available reformers use
alcohols, such as methanol and ethanol, hydrocarbons, such as methane,
propane, and butane, and liquid fuels, such as gasoline and light oil, as
reforming materials.
In all the embodiments discussed above, the oxygen-containing oxidizing gas
is the air. The principle of the present invention may also be applicable
to pure oxygen.
The present invention is not restricted to the above embodiments or their
modified examples, but there may be many other modifications, changes, and
alterations without departing from the scope or spirit of the main
characteristics of the present invention.
It should be clearly understood that the above embodiments are only
illustrative and not restrictive in any sense. The scope and spirit of the
present invention are limited only by the terms of the appended claims.
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